This invention relates to an opto-electronic memory system comprising a planar memory element in which information is stored in a two dimensional array of optically readable memory locations and reading means for optically reading information in the memory element and providing an electrical output in accordance therewith, the reading means comprising an electro-optic shutter arrangement which is operable to address memory locations of the memory element with reading light and to scan the array with said reading light along a first direction of the array, and a light sensing arrangement comprising a planar array of linear light sensitive elements disposed adjacent to the memory element with the linear light sensitive elements extending parallel to one another in said first direction for sensing light from the memory element.
Such a system is described in EP-A-0509597. In this system, the electro-optic shutter arrangement consists of a planar array of linear liquid crystal light shutters arranged parallel to one another and extending in a second direction perpendicular to the first direction, together with a light source which illuminates the array over its input side. The memory element, in the form, for example, of a digital data card, carries optically encoded data and consists of a medium, such as a photographic medium, having memory locations arranged in rows and columns whose light transmission characteristics denote stored information. To read the stored information the memory element is disposed in a gap between the array of electro-optic shutters and the array of light sensitive elements, which consists of thin film photodiodes in strip form carried on a support. The liquid crystal light shutters are operated individually in sequence so as in effect to scan in step fashion a band of light determined by the size of a light shutter and covering the height of a column of memory locations in the memory element across the array in the row direction. The system may also be operated in a random access mode wherein the light shutters are operated to scan the memory element in a non-sequential manner. The system can be used for reading stored digital data in applications similar to CD-ROM systems, audio CD systems and so-called laser card systems. In these systems information consisting of digital data stored in an optically-readable manner on a carrier such as a disc or card is read out by electromechanically scanning a light beam over the carrier and detecting the beam reflected or transmitted in accordance with stored information with the carrier being scanned by moving the carrier relative to the light beam and detector.
Unlike these known systems, the system of EP-A-0509597 uses an entirely solid-state optical scanning approach and requires no moving parts for a read operation. The avoidance of an electro-mechanical scanning arrangement, typically involving motors and optical components such as lenses, offers significant advantages. Most importantly, the reading means can be more compact with the arrays of electro-optic shutters and light sensitive elements occupying an area approximately corresponding to the information storage area of the memory element. In the system of EP-A-0509597 however, it is important for successful operation that the light directed towards the memory element be highly collimated so that the light entering the memory element is substantially parallel and is directed substantially perpendicularly to the plane of the memory element, particularly if a high density of memory locations is used. The light shutter array should ideally be very close to the light sensitive element array. However, because the memory element is disposed between these two components a certain minimum thickness of the gap between the output side of the former and the input side of the latter into which the memory element is inserted is demanded, this minimum thickness being determined by the thickness of the memory element. In relation to the thicknesses of the components of the reading means this can be significant. Highly parallel light directed perpendicularly onto the light shutter array is thus required to avoid problems of cross-talk. Moreover, after a period of time during which memory elements are repeatedly inserted and extracted, dust and other deposits can accumulate in the gap which may affect reading performance. Because the light shutter array and the sensing element array are fixed together forming an assembly, cleaning of the gap between these two components, and particularly their exposed surfaces, is difficult.